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<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="statements.html" title="cpp/language/statements"> Statements</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Labels </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">label</span> : <span class="t-spar">statement</span></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Expression statements </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> <span class="t-spar">expression</span> ; </a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Compound statements </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="statements.html" title="cpp/language/statements"> { <span class="t-spar">statement</span>... } </a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Selection statements </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> if</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="switch.html" title="cpp/language/switch"> switch</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Iteration statements </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="while.html" title="cpp/language/while"> while</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="do.html" title="cpp/language/do"> do-while</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="for.html" title="cpp/language/for"> for</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="range-for.html" title="cpp/language/range-for"> range for</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Jump statements </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="break.html" title="cpp/language/break"> break</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="continue.html" title="cpp/language/continue"> continue</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="return.html" title="cpp/language/return"> return</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="goto.html" title="cpp/language/goto"> goto</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Declaration statements </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> <span class="t-spar">declaration</span> ; </a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Try blocks </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="try_catch.html" title="cpp/language/try catch"> try <span class="t-spar">compound-statement</span> <span class="t-spar">handler-sequence</span></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Transactional memory </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="transactional_memory.html" title="cpp/language/transactional memory"> <code>synchronized</code>, <code>atomic_commit</code></a>, etc<span class="t-mark-rev t-since-tm-ts t-mark-ts">(TM TS)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Conditionally executes another statement.
</p><p>Used where code needs to be executed based on a run-time<span class="t-rev-inl t-since-cxx17"><span> or compile-time</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> condition<span class="t-rev-inl t-since-cxx23"><span>, or whether the if statement is evaluated in a manifestly constant-evaluated context</span> <span><span class="t-mark-rev t-since-cxx23">(since C++23)</span></span></span>.
</p>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(optional)</span> <code><b>(</b></code> <span class="t-spar">init-statement</span><span class="t-mark">(optional)</span> <span class="t-spar">condition</span> <code><b>)</b></code> <span class="t-spar">statement-true</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(optional)</span> <code><b>(</b></code> <span class="t-spar">init-statement</span><span class="t-mark">(optional)</span> <span class="t-spar">condition</span> <code><b>)</b></code> <span class="t-spar">statement-true</span> <code><b>else</b></code> <span class="t-spar">statement-false</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>




<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>!</b></code><span class="t-mark">(optional)</span> <code><b>consteval</b></code> <span class="t-spar">compound-statement</span>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx23">(since C++23)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>!</b></code><span class="t-mark">(optional)</span> <code><b>consteval</b></code> <span class="t-spar">compound-statement</span> <code><b>else</b></code> <span class="t-spar">statement</span>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx23">(since C++23)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> <i>if statement</i>  without an else branch</div>
<div class="t-li1"><span class="t-li">2)</span> <i>if statement</i>  with an else branch</div>
<div class="t-li1"><span class="t-li">3)</span> <i>consteval if statement</i>  without an else branch</div>
<div class="t-li1"><span class="t-li">4)</span> <i>consteval if statement</i>  with an else branch</div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <code><b>constexpr</b></code>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span> if present, the statement becomes a <i>constexpr if statement</i>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">init-statement</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span> either
<ul><li> an <a href="statements.html" title="cpp/language/statements">expression statement</a> (which may be a <i>null statement</i> "<code><b>;</b></code>")
</li><li> a  <a href="declarations.html" title="cpp/language/declarations">simple declaration</a>, typically a declaration of a variable with initializer, but it may declare arbitrary many variables or be a <a href="structured_binding.html" title="cpp/language/structured binding">structured binding</a> declaration
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx23"><td>
<ul><li> an <a href="type_alias.html" title="cpp/language/type alias">alias declaration</a>
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx23">(since C++23)</span></td></tr>
</table>
<dl><dd>Note that any <span class="t-spar">init-statement</span> must end with a semicolon <code><b>;</b></code>, which is why it is often described informally as an expression or a declaration followed by a semicolon.
</dd></dl>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">condition</span>
</td>
<td> -
</td>
<td> one of
<ul><li> <a href="expressions.html" title="cpp/language/expressions">expression</a> which is <a href="implicit_cast.html#Contextual_conversions" title="cpp/language/implicit conversion">contextually convertible</a> to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>
</li><li> <a href="declarations.html" title="cpp/language/declarations">declaration</a> of a single non-array variable with a brace-or-equals <a href="initialization.html" title="cpp/language/initialization">initializer</a>.
</li></ul>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">statement-true</span>
</td>
<td> -
</td>
<td> any <a href="statements.html" title="cpp/language/statements">statement</a> (often a compound statement), which is executed if <span class="t-spar">condition</span> evaluates to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">statement-false</span>
</td>
<td> -
</td>
<td> any <a href="statements.html" title="cpp/language/statements">statement</a> (often a compound statement), which is executed if <span class="t-spar">condition</span> evaluates to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">compound-statement</span>
</td>
<td> -
</td>
<td> any <a href="statements.html#Compound_statements" title="cpp/language/statements">compound statement</a>, which is executed if the if-statement
<ul><li> is evaluated in a <a href="constant_expression.html#Manifestly_constant-evaluated_expressions" title="cpp/language/constant expression">manifestly constant-evaluated context</a>, if <code>!</code> is not preceding <code>consteval</code>
</li><li> is not evaluated in a manifestly constant-evaluated context, if <code>!</code> is preceding <code>consteval</code>
</li></ul>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">statement</span>
</td>
<td> -
</td>
<td> any <a href="statements.html" title="cpp/language/statements">statement</a> (must be a compound statement, see <a href="if.html#Constexpr_if" title="cpp/language/if">below</a>), which is executed if the if-statement
<ul><li> is not evaluated in a <a href="constant_expression.html#Manifestly_constant-evaluated_expressions" title="cpp/language/constant expression">manifestly constant-evaluated context</a>, if <code>!</code> is not preceding <code>consteval</code>
</li><li> is evaluated in a manifestly constant-evaluated context, if <code>!</code> is preceding <code>consteval</code>
</li></ul>
</td></tr></table>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>If the <span class="t-spar">condition</span> yields <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> after conversion to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>, <span class="t-spar">statement-true</span> is executed.
</p><p>If the else part of the if statement is present and <span class="t-spar">condition</span> yields <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> after conversion to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>, <span class="t-spar">statement-false</span> is executed.
</p><p>In the second form of if statement (the one including else), if <span class="t-spar">statement-true</span> is also an if statement then that inner if statement must contain an else part as well (in other words, in nested if-statements, the else is associated with the closest if that doesn't have an else)
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// simple if-statement with an else clause</span>
    <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">&gt;</span> <span class="nu0">2</span><span class="br0">)</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" is greater than 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="kw1">else</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" is not greater than 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    <span class="co1">// nested if-statement</span>
    <span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">&gt;</span> <span class="nu0">1</span><span class="br0">)</span>
        <span class="kw1">if</span> <span class="br0">(</span>j <span class="sy1">&gt;</span> <span class="nu0">2</span><span class="br0">)</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 1 and "</span> <span class="sy1">&lt;&lt;</span> j <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
        <span class="kw1">else</span> <span class="co1">// this else is part of if (j &gt; 2), not of if (i &gt; 1)</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">" &gt; 1 and "</span> <span class="sy1">&lt;&lt;</span> j <span class="sy1">&lt;&lt;</span> <span class="st0">" &lt;= 2<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    <span class="co1">// declarations can be used as conditions with dynamic_cast</span>
    <span class="kw1">struct</span> Base
    <span class="br0">{</span>
        <span class="kw1">virtual</span> ~Base<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">struct</span> Derived <span class="sy4">:</span> Base
    <span class="br0">{</span>
        <span class="kw4">void</span> df<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"df()<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
 
    Base<span class="sy2">*</span> bp1 <span class="sy1">=</span> new Base<span class="sy4">;</span>
    Base<span class="sy2">*</span> bp2 <span class="sy1">=</span> new Derived<span class="sy4">;</span>
 
    <span class="kw1">if</span> <span class="br0">(</span>Derived<span class="sy2">*</span> p <span class="sy1">=</span> <span class="kw1">dynamic_cast</span><span class="sy1">&lt;</span>Derived<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span>bp1<span class="br0">)</span><span class="br0">)</span> <span class="co1">// cast fails, returns nullptr</span>
        p<span class="sy2">-</span><span class="sy1">&gt;</span>df<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// not executed</span>
 
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> p <span class="sy1">=</span> <span class="kw1">dynamic_cast</span><span class="sy1">&lt;</span>Derived<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span>bp2<span class="br0">)</span><span class="br0">)</span> <span class="co1">// cast succeeds</span>
        p<span class="sy2">-</span><span class="sy1">&gt;</span>df<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// executed</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2 is not greater than 2
2 &gt; 1 and 1 &lt;= 2
df()</pre></div></div> 
</div>
<p><br>
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h4> <span class="mw-headline" id="If_statements_with_initializer"> If statements with initializer </span></h4>
<p>If <span class="t-spar">init-statement</span> is used, the if statement is equivalent to
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>{</b></code><br>
<dl><dd><span class="t-spar">init_statement</span> <br>
</dd><dd><span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(optional)</span> <code><b>(</b></code> <span class="t-spar">condition</span> <code><b>)</b></code>  <br>
<dl><dd><span class="t-spar">statement-true</span> <br>
</dd></dl>
</dd></dl>
<p><code><b>}</b></code>
</p>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>or 
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>{</b></code><br>
<dl><dd><span class="t-spar">init_statement</span><br>
</dd><dd><span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>if</b></code> <code><b>constexpr</b></code><span class="t-mark">(optional)</span> <code><b>(</b></code> <span class="t-spar">condition</span> <code><b>)</b></code> <br>
<dl><dd><span class="t-spar">statement-true</span><br>
</dd></dl>
</dd><dd><code><b>else</b></code>
<dl><dd><span class="t-spar">statement-false</span><br>
</dd></dl>
</dd></dl>
<p><code><b>}</b></code>
</p>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>Except that names declared by the <span class="t-spar">init-statement</span> (if <span class="t-spar">init-statement</span> is a declaration) and names declared by <span class="t-spar">condition</span> (if condition is a declaration) are in the same scope, which is also the scope of both <span class="t-spar">statement</span>s.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../container/map.html"><span class="kw1284">std::<span class="me2">map</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> m<span class="sy4">;</span>
<a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a> mx<span class="sy4">;</span>
<span class="kw4">extern</span> <span class="kw4">bool</span> shared_flag<span class="sy4">;</span> <span class="co1">// guarded by mx</span>
 
<span class="kw4">int</span> demo<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> it <span class="sy1">=</span> m.<span class="me1">find</span><span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> it <span class="sy3">!</span><span class="sy1">=</span> m.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> it<span class="sy2">-</span><span class="sy1">&gt;</span>second.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">char</span> buf<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span> <a href="../io/c/fgets.html"><span class="kw1827">std::<span class="me2">fgets</span></span></a><span class="br0">(</span>buf, <span class="nu0">10</span>, <span class="kw2">stdin</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> m<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy2">+</span><span class="sy1">=</span> buf<span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span><a href="../thread/lock_guard.html"><span class="kw2170">std::<span class="me2">lock_guard</span></span></a> lock<span class="br0">(</span>mx<span class="br0">)</span><span class="sy4">;</span> shared_flag<span class="br0">)</span> <span class="br0">{</span> unsafe_ping<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> shared_flag <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">int</span> s<span class="sy4">;</span> <span class="kw4">int</span> count <span class="sy1">=</span> ReadBytesWithSignal<span class="br0">(</span><span class="sy3">&amp;</span>s<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> publish<span class="br0">(</span>count<span class="br0">)</span><span class="sy4">;</span> raise<span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> keywords <span class="sy1">=</span> <span class="br0">{</span><span class="st0">"if"</span>, <span class="st0">"for"</span>, <span class="st0">"while"</span><span class="br0">}</span><span class="sy4">;</span>
        std<span class="sy4">::</span><a href="../algorithm/ranges/all_any_none_of.html"><span class="kw2851">ranges::<span class="me2">any_of</span></span></a><span class="br0">(</span>keywords, <span class="br0">[</span><span class="sy3">&amp;</span>tok<span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> kw<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> tok <span class="sy1">==</span> kw<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="br0">)</span>
    <span class="br0">{</span>
        <a href="../io/cerr.html"><span class="kw1764">std::<span class="me2">cerr</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Token must not be a keyword<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h4> <span class="mw-headline" id="Constexpr_if">Constexpr if</span></h4>
<p>The statement that begins with <code><b>if constexpr</b></code> is known as the <i>constexpr if statement</i>.
</p><p>In a constexpr if statement, the value of <span class="t-spar">condition</span> must be <span class="t-rev-inl t-until-cxx23"><span>a <a href="constant_expression.html#Converted_constant_expression" title="cpp/language/constant expression">contextually converted constant expression of type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></a></span> <span><span class="t-mark-rev t-until-cxx23">(until C++23)</span></span></span><span class="t-rev-inl t-since-cxx23"><span>an expression <a href="implicit_cast.html#Contextual_conversions" title="cpp/language/implicit conversion">contextually converted to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></a>, where the conversion is a <a href="constant_expression.html" title="cpp/language/constant expression">constant expression</a></span> <span><span class="t-mark-rev t-since-cxx23">(since C++23)</span></span></span>. If the value is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>, then <span class="t-spar">statement-false</span> is discarded (if present), otherwise, <span class="t-spar">statement-true</span> is discarded. 
</p><p>The return statements in a discarded statement do not participate in function return type deduction:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">auto</span> get_value<span class="br0">(</span>T t<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_pointer.html"><span class="kw469">std::<span class="me2">is_pointer_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
        <span class="kw1">return</span> <span class="sy2">*</span>t<span class="sy4">;</span> <span class="co1">// deduces return type to int for T = int*</span>
    <span class="kw1">else</span>
        <span class="kw1">return</span> t<span class="sy4">;</span>  <span class="co1">// deduces return type to int for T = int</span>
<span class="br0">}</span></pre></div></div>
<p>The discarded statement can <a href="definition.html#One_Definition_Rule" title="cpp/language/definition">odr-use</a> a variable that is not defined
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// no definition of x required</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span>
        <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
    <span class="kw1">else</span> <span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span>
        <span class="kw1">return</span> x<span class="sy4">;</span>
    <span class="kw1">else</span>
        <span class="kw1">return</span> <span class="sy2">-</span>x<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Outside a template, a discarded statement is fully checked. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="kw4">constexpr</span></span></span> is not a substitute for the <a href="../preprocessor/conditional.html" title="cpp/preprocessor/conditional"><tt>
#if</tt></a> preprocessing directive:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span><span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span>
    <span class="br0">{</span>
        <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
        <span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// Error even though in discarded statement</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>If a constexpr if statement appears inside a <a href="templates.html#Templated_entity" title="cpp/language/templates">templated entity</a>, and if <span class="t-spar">condition</span> is not <a href="dependent_name.html#Value-dependent_expressions" title="cpp/language/dependent name">value-dependent</a> after instantiation, the discarded statement is not instantiated when the enclosing template is instantiated . 
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T, <span class="kw1">typename</span> ... <span class="me1">Rest</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> g<span class="br0">(</span>T<span class="sy3">&amp;&amp;</span> p, Rest<span class="sy3">&amp;&amp;</span> ...<span class="me1">rs</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// ... handle p</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>sizeof...<span class="br0">(</span>rs<span class="br0">)</span> <span class="sy1">&gt;</span> <span class="nu0">0</span><span class="br0">)</span>
        g<span class="br0">(</span>rs...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// never instantiated with an empty argument list.</span>
<span class="br0">}</span></pre></div></div>
<p>Note: an example where the condition remains value-dependent after instantiation is a nested template, e.g.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">auto</span> lm <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> p<span class="br0">)</span>
    <span class="br0">{</span>
        <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>sizeof<span class="br0">(</span>T<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">1</span> <span class="sy3">&amp;&amp;</span> sizeof p <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span>
        <span class="br0">{</span>
            <span class="co1">// this condition remains value-dependent after instantiation of g&lt;T&gt;</span>
        <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: the status seems to be changed by <a rel="nofollow" class="external text" href="https://wg21.link/P0588R1">P0588R1</a>. </td></tr></table>
<p>Note: the discarded statement can't be ill-formed for every possible specialization:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_arithmetic.html"><span class="kw489">std::<span class="me2">is_arithmetic_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
        <span class="co1">// ...</span>
    <span class="kw1">else</span>
        static_assert<span class="br0">(</span><span class="kw2">false</span>, <span class="st0">"Must be arithmetic"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ill-formed: invalid for every T</span>
<span class="br0">}</span></pre></div></div>
<p>The common workaround for such a catch-all statement is a type-dependent expression that is always false:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span><span class="sy1">&gt;</span>
<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> dependent_false_v <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="../types/is_arithmetic.html"><span class="kw489">std::<span class="me2">is_arithmetic_v</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">)</span>
        <span class="co1">// ...</span>
    <span class="kw1">else</span>
        static_assert<span class="br0">(</span>dependent_false_v<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span>, <span class="st0">"Must be arithmetic"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ok</span>
<span class="br0">}</span></pre></div></div>
<p>Labels (<a href="goto.html" title="cpp/language/goto">goto targets</a>, <code>case</code> labels, and <code>default:</code>) appearing in a substatement of a constexpr if can only be referenced (by <a href="switch.html" title="cpp/language/switch"><code>switch</code></a> or <a href="goto.html" title="cpp/language/goto"><code>goto</code></a>) in the same substatement.
</p><p>Note: a <a href="typedef.html" title="cpp/language/typedef">typedef declaration</a> <span class="t-rev-inl t-since-cxx23"><span>or <a href="type_alias.html" title="cpp/language/type alias">alias declaration</a></span> <span><span class="t-mark-rev t-since-cxx23">(since C++23)</span></span></span> can be used as the init-statement of a constexpr if statement to reduce the scope of the type alias.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: no example </td></tr></table>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx23"><td>
<h4> <span class="mw-headline" id="Consteval_if">Consteval if</span></h4>
<p>The statement that begins with <code><b>if consteval</b></code> is known as the <i>consteval if statement</i>. In a consteval if statement, both <span class="t-spar">compound-statement</span> and <span class="t-spar">statement</span> (if any) must be compound statements.
</p><p>If <span class="t-spar">statement</span> is not a compound statement, it will still be treated as a part of the consteval if statement (and thus results in a compilation error):
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">constexpr</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">bool</span> b<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span>
        <span class="kw1">if</span> consteval <span class="br0">{</span><span class="br0">}</span>
        <span class="kw1">else</span> <span class="sy4">;</span> <span class="co1">// error: not a compound-statement</span>
               <span class="co1">// else not associated with outer if</span>
<span class="br0">}</span></pre></div></div>
</div>
<p>If a consteval if statement is evaluated in a <a href="constant_expression.html#Manifestly_constant-evaluated_expressions" title="cpp/language/constant expression">manifestly constant-evaluated context</a>, <span class="t-spar">compound-statement</span> is executed. Otherwise, <span class="t-spar">statement</span> is executed if it is present.
</p><p>A <code><b>case</b></code> or <code><b>default</b></code> label appearing within a consteval if statement shall be associated with a <a href="switch.html" title="cpp/language/switch"><code>switch</code></a> statement within the same if statement. A <a href="goto.html" title="cpp/language/goto">label</a> declared in a substatement of a consteval if statement shall only be referred to by a statement in the same substatement.
</p><p>If the statement begins with <code><b>if !consteval</b></code>, the <span class="t-spar">compound-statement</span> and <span class="t-spar">statement</span> (if any) must be both compound statements. Such statement is not considered as consteval if statement, but is equivalent to a consteval if statement:
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="sy3">!</span>consteval <span class="br0">{</span><span class="coMULTI">/*stmt*/</span><span class="br0">}</span></span></span> is equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> consteval <span class="br0">{</span><span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span><span class="coMULTI">/*stmt*/</span><span class="br0">}</span></span></span>.
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> <span class="sy3">!</span>consteval <span class="br0">{</span><span class="coMULTI">/*stmt-1*/</span><span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span><span class="coMULTI">/*stmt-2*/</span><span class="br0">}</span></span></span> is equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span> consteval <span class="br0">{</span><span class="coMULTI">/*stmt-2*/</span><span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span><span class="coMULTI">/*stmt-1*/</span><span class="br0">}</span></span></span>.
</li></ul>
<p><span class="t-spar">compound-statement</span> in a consteval if statement (or <span class="t-spar">statement</span> in the negative form) is in an <a href="consteval.html" title="cpp/language/consteval">immediate function context</a>, in which a call to an immediate function needs not to be a constant expression.
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cmath&gt;</span>
<span class="co2">#include &lt;cstdint&gt;</span>
<span class="co2">#include &lt;cstring&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">constexpr</span> <span class="kw4">bool</span> is_constant_evaluated<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span>
<span class="br0">{</span>
    <span class="kw1">if</span> consteval <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span>
 
<span class="kw4">constexpr</span> <span class="kw4">bool</span> is_runtime_evaluated<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span>
<span class="br0">{</span>
    <span class="kw1">if</span> not consteval <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span>
 
consteval <a href="../types/integer.html"><span class="kw129">std::<span class="me2">uint64_t</span></span></a> ipow_ct<span class="br0">(</span><a href="../types/integer.html"><span class="kw129">std::<span class="me2">uint64_t</span></span></a> base, <a href="../types/integer.html"><span class="kw126">std::<span class="me2">uint8_t</span></span></a> exp<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="sy3">!</span>base<span class="br0">)</span> <span class="kw1">return</span> base<span class="sy4">;</span>
    <a href="../types/integer.html"><span class="kw129">std::<span class="me2">uint64_t</span></span></a> res<span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="kw1">while</span> <span class="br0">(</span>exp<span class="br0">)</span>
    <span class="br0">{</span>
        <span class="kw1">if</span> <span class="br0">(</span>exp <span class="sy3">&amp;</span> <span class="nu0">1</span><span class="br0">)</span> res <span class="sy2">*</span><span class="sy1">=</span> base<span class="sy4">;</span>
        exp <span class="sy2">/</span><span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
        base <span class="sy2">*</span><span class="sy1">=</span> base<span class="sy4">;</span>
    <span class="br0">}</span>
    <span class="kw1">return</span> res<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">constexpr</span> <a href="../types/integer.html"><span class="kw129">std::<span class="me2">uint64_t</span></span></a> ipow<span class="br0">(</span><a href="../types/integer.html"><span class="kw129">std::<span class="me2">uint64_t</span></span></a> base, <a href="../types/integer.html"><span class="kw126">std::<span class="me2">uint8_t</span></span></a> exp<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">if</span> consteval <span class="co1">// use a compile-time friendly algorithm</span>
    <span class="br0">{</span>
        <span class="kw1">return</span> ipow_ct<span class="br0">(</span>base, exp<span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
    <span class="kw1">else</span> <span class="co1">// use runtime evaluation</span>
    <span class="br0">{</span>
        <span class="kw1">return</span> <a href="../numeric/math/pow.html"><span class="kw1335">std::<span class="me2">pow</span></span></a><span class="br0">(</span>base, exp<span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> argv<span class="br0">[</span><span class="br0">]</span><span class="br0">)</span>
<span class="br0">{</span>
    static_assert<span class="br0">(</span>ipow<span class="br0">(</span><span class="nu0">0</span>,<span class="nu0">10</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span> <span class="sy3">&amp;&amp;</span> ipow<span class="br0">(</span><span class="nu0">2</span>,<span class="nu0">10</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">1024</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> ipow<span class="br0">(</span><a href="../string/byte/strlen.html"><span class="kw1148">std::<span class="me2">strlen</span></span></a><span class="br0">(</span>argv<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span>, <span class="nu0">3</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</div>
</td>
<td><span class="t-mark-rev t-since-cxx23">(since C++23)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>If <span class="t-spar">statement-true</span> or <span class="t-spar">statement-false</span> is not a compound statement, it is treated as if it were:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="co1">// i is no longer in scope</span></pre></div></div>
<p>is the same as
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span>x<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span>
<span class="co1">// i is no longer in scope</span></pre></div></div>
<p>The scope of the name introduced by <span class="t-spar">condition</span>, if it is a declaration, is the combined scope of both statements' bodies:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">if</span> <span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// error: redeclaration of x</span>
<span class="br0">}</span>
<span class="kw1">else</span>
<span class="br0">{</span>
    <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// error: redeclaration of x</span>
<span class="br0">}</span></pre></div></div>
<p>If <span class="t-spar">statement-true</span> is entered by <a href="goto.html" title="cpp/language/goto"><code>goto</code></a> or <span class="t-lc">longjmp</span>, <span class="t-spar">condition</span> is not evaluated and <span class="t-spar">statement-false</span> is not executed.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17 t-until-cxx23"><td>
<p>Built-in conversions are not allowed in the <span class="t-spar">condition</span> of a constexpr if statment, except for non-<a href="list_initialization.html#Narrowing_conversions" title="cpp/language/list initialization">narrowing</a> <a href="implicit_cast.html#Integral_conversions" title="cpp/language/implicit conversion">integral conversions</a> to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span><br><span class="t-mark-rev t-until-cxx23">(until C++23)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p><a href="switch.html" title="cpp/language/switch"><code>switch</code></a> and <a href="goto.html" title="cpp/language/goto"><code>goto</code></a> are not allowed to jump into a branch of constexpr if statement<span class="t-rev-inl t-since-cxx23"><span> or a consteval if statement</span> <span><span class="t-mark-rev t-since-cxx23">(since C++23)</span></span></span>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keywords/if.html" title="cpp/keyword/if"><tt>if</tt></a>,
<a href="../keyword/else.html" title="cpp/keyword/else"><tt>else</tt></a>,
<a href="../keyword/constexpr.html" title="cpp/keyword/constexpr"><tt>constexpr</tt></a>,
<a href="../keyword/consteval.html" title="cpp/keyword/consteval"><tt>consteval</tt></a>
</p>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/631.html">CWG 631</a>
</td>
<td> C++98
</td>
<td> the control flow was unspecified if the<br>first substatement is reached via a label
</td>
<td> the condition is not evaluated and the second<br>substatement is not executed (same as in C)
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../types/is_constant_evaluated.html" title="cpp/types/is constant evaluated"> <span class="t-lines"><span>is_constant_evaluated</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div>
</td>
<td>   detects whether the call occurs within a constant-evaluated context  <br> <span class="t-mark">(function)</span> </td></tr>

</table>

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